A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. However, much of the highly energetic X-radiation simply passes through the radiographic element without being absorbed. To reduce patient exposure to X-radiation it is conventional practice in medical radiology to employ silver halide radiographic elements in combination with intensifying screens, where the intensifying screen contains a phosphor layer that absorbs X-radiation more efficiently than silver halide and emits longer wavelength electromagnetic radiation which silver halide can more efficiently absorb.
Intensifying screens that emit in the ultraviolet or visible portion of the spectrum are generally known, as illustrated by Research Disclosure, Vol. 184, Aug. 1979, Item 18431, particularly Sections IX and XI. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
Green emitting intensifying screens have found widespread use with silver halide radiographic elements. To absorb light in the green portion of the spectrum silver halide emulsions must be spectrally sensitized by adsorbing one or more spectral sensitizing dyes to the surfaces of the silver halide grains in the emulsions. Although routine, spectral sensitization is not without its disadvantages. The dyes themselves are complex organic molecules that, on a weight basis, are more expensive than silver, but, unlike silver, are not recoverable for reuse. Further, emulsion addenda that also adsorb to grain surfaces, such as antifoggants and stabilizers, can displace the dyes, leading to reduced spectral sensitivity.
Blue emitting intensifying screens capable of imagewise exposing silver halide radiographic elements within the blue (400 to 500 nm) region of the spectrum are known in the art. Although many blue emitting phosphors are known, calcium tungstate has for many years been the standard blue emitting phosphor for use in intensifying screens against which blue emitting intensifying screens have been compared.
In the blue region of the spectrum silver halides exhibit sharply declining light absorption as a function of increasing wavelengths. James The Theory of the Photographic Process, 4th Ed., Macmillan, New York, 1977, FIG. 1.16, p. 39, shows the relative absorptions of representative photographically useful silver halides as a function of wavelength. From FIG. 1.16 it is apparent that adsorption properties of silver halides in the blue portion of the spectrum are in transition from very high levels of absorption that occur in the near ultraviolet (220 to 400 nm) portion of the spectrum to the very low levels of absorption that occur in the green region of the spectrum. Thus, spectral sensitizing dyes are often used with silver halide emulsions intended to be exposed with blue emitting phosphors (note Research Disclosure, Item 18431, cited above, Section X), and this results in the same disadvantages encountered in using green emitting intensifying screens.
In addition to or instead of using a blue absorbing spectral sensitizing dye, it is also common to incorporate iodide into the silver halide emulsions to enhance their sensitivity within the blue portion of the spectrum. Although iodide in low levels is commonly employed in photography, there are distinct disadvantages. When iodide is incorporated in silver halide emulsions, the rate of processing is slowed and the frequency with which processing solutions must be replenished is increased. The former disadvantage is particularly troublesome in radiographic imaging, since total processing must be accomplished in less than 90 seconds and, preferably, less than one minute. The latter disadvantage has become of increasing concern as environment protecting regulations on spent processing solutions have become progressively more demanding.
In most medical applications for X-ray imaging a pair of intensifying screens are employed in combination with a radiographic film having silver halide emulsion layer units coated on opposite sides of the support. This arrangement makes maximum use of the X-radiation available for imaging. Unfortunately, unless additional structural features- are added, this arrangement also has the disadvantage of reduced image sharpness attributable to crossover. Crossover results when light emitted by one intensifying screen penetrates the adjacent silver halide layer unit and crosses through the support to also expose the silver halide emulsion unit on the opposite side of the support. Eliminating crossover involves incorporating additional light intercepting layers in the radiographic element, thereby complicating its construction and/or processing.
Although it has been recognized that silver halide possesses more native sensitivity to ultraviolet radiation than to visible light and although it has been recognized that, in principle, X-ray intensifying screens can be constructed to emit ultraviolet light, the fact is that the main thrust of X-ray intensifying screen and film imaging system development has been toward those imaging systems that employ visible light for screen emission and film exposure. One reason for this approach is that the development of silver halide radiographic films has been greatly influenced by the parallel development of silver halide photographic elements, which are necessarily responsive to visible light. Another reason lies in the fact that the organic materials (e.g., binders, hydrophilic colloid vehicles, film supports, etc.) exhibit high levels of absorption of shorter wavelengths of ultraviolet radiation.
Kroger et al U.S. Pat. No. 2,542,336 discloses phosphors containing titanium as an activator and having a matrix comprised of one or more of the oxides of zirconium, hafnium, thorium, germanium or tin to which may be added either acid oxides or basic oxides or both. Disclosed basic oxides are the oxides of sodium, potassium, rubidium, cesium, lithium, barium, calcium, strontium, magnesium, beryllium and zinc. Disclosed acid oxides are SO.sub.3, B.sub.2 O.sub.3, P.sub.2 O.sub.5 and SiO.sub.2. Titanium activated zirconium oxide, magnesium stannate, calcium zirconate and zirconium phosphate are each specifically disclosed.
Titanium activated germanium oxide is a blue emitting phosphor, but investigations have revealed titanium activated germanium oxide to exhibit low emission intensities.
Titanium activated hafnium oxide exhibits peak emission in the longer wavelength (approx. 475 nm) blue portion of the spectrum, with a substantial portion of its total emission extending into the green region of the spectrum. L. H. Brixner, "Structural and Luminescent Properties of the Ln.sub.2 Hf.sub.2 O.sub.7 -type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, after reporting the properties of Ti.sup.+4 as an activator for rare earth hafnates, noted a high level of performance for titanium activated optical grade hafnia (HfO.sub.2), but considered the phosphor impractical for intensifying screen use based on the price of optical grade hafnia. Optical grade hafnia contains less than 3.times.10.sup.-4 mole of zirconia (ZrO.sub.2) per mole of hafnia.
Bryan et al U.S. Pat. No. 4,988,880 discloses that efficient X-ray intensifying screens can be constructed from titanium activated hafnia phosphors containing minor amounts of zirconium, but higher amounts than found in optical grade hafnia, specifically: EQU Hf.sub.1-z Zr.sub.z
where
z ranges from 4.times.10.sup.-4 to 0.3. Sharp losses in emission intensities were found at higher values of z. The same phosphor, but lacking a titanium activator, is also disclosed.
Phosphors which contain germanium, zirconium or hafnium and oxygen with oxygen being complexed with other nonmetals, such as sulfur, boron, phosphorus, silicon and the like, produce distinctly different crystal structures than those of hafnium and/or zirconium germanate and are not considered relevant to this invention.